Forty adults with Down syndrome will be compared to two control groups matched on age and gender to test aging effects of contrast sensitivity. Control groups will include 40 adults with unknown-caused mental retardation and 40 nonretarded adults. Each group will be divided into a young (18-25 years) and old (35-45 years) age group. Subjects will be given a battery of tests which will include verbal reasoning, visual acuity, contrast sensitivity, and cognitive abilities. Tests will be repeated after six-months, and will be administered at a lab on campus. There are no health risks beyond daily living. Confidentiality will be maintained by subject number assignment, and informed consent are obtained before testing. There are two main aims of this study. The first aim is to study effects of aging in contrast sensitivity. Contrast sensitivity is measured at 5 spatial frequencies (.5,1, 2, 4, 8 cycles/degree) and 2 temporal frequencies (0 hertz, 7.5 hertz). Contrast sensitivity is found to decline with age and suggested to be associated with neuropathogenesis of visual pathways in individuals with Alzheimer's disease. It is hypothesized that subjects with Down syndrome will have lower contrast sensitivity and progressively decline over time compared to their controls. Furthermore, the older subjects with Down syndrome will have a lower contrast sensitivity function compared to the younger subjects with Down syndrome. The second aim is to study the contrast sensitivity effects on cognitive abilities. Two computerized cognitive tests will be administered. The first cognitive test is a simple reaction time task. Subjects touch a stimulus on a touch-screen when it lights up. Decision and movement times will be recorded. The second test is a match-to-sample task to test identification. Subjects are to match a probe with six stimuli. Reaction time, decision time, and percent correct will be recorded. Visual stimuli of the cognitive tests are manipulated in contrast. Each cognitive test consists of three contrast conditions (normal, high, & low). The high condition enhances the stimuli in contrast to 90%, whereas the low condition decreases the stimuli in contrast to 30%. Subjects will be given all conditions in counterbalanced order. It is hypothesized that performance on cognitive tasks will decline at the lower and normal contrast conditions and improve at the higher contrast condition for subjects with Down syndrome compared to their controls. Analysis of variance for repeated measures will be used for the purposes of this study to analyze changes in contrast sensitivity over time and cognitive performance at contrast conditions.